1. Field of the Invention
The present invention relates to a technique of recording dots on a printing medium with a dot-recording head.
2. Description of the Related Art
Ink jet printers such as serial scan-type printers and drum scan-type printers are dot recording apparatus that record dots on a printing medium with a dot-recording head while carrying out scans both in a main scanning direction and in a sub-scanning direction. An ink jet printer has a plurality of nozzles formed on a print head to spout ink and thereby print characters and images on a printing medium. Each nozzle on a print head has a pressure chamber filled with ink and an electrical-to-mechanical conversion element. Application of an electric signal to the electrical-to-mechanical conversion element produces a pressure in the pressure chamber and causes ink droplets to jet out from the nozzle.
Picture quality improvement has been one of major issues for ink jet printers. One proposed technique is the "interlace printing" disclosed in U.S. Pat. No. 4,198,642. FIG. 16 shows a conventional interlace printing scheme. A print head 1 has eleven nozzles #1-#11. A pitch k of the nozzles in the sub-scanning direction is set equal to 4 dots. Here the unit [dot] is defined as a minimum pitch P [inch] of dots in the sub-scanning direction recorded on the printing medium, and thus k dots correspond to k.times.P inches. In FIG. 16, the position of the print head 1 shown as pass 1, pass 2, or the like represents the position in the sub-scanning direction in each main scan. The term "pass" means one main scan. After each main scan, a sub-scan feed is carried out by a fixed amount F of 11 dots.
In the conventional interlace printing, the following two conditions are set to prevent skipping and overwriting of main scanning lines (hereinafter also referred to as "raster lines"):
[Condition 1] Number of used nozzles N and nozzle pitch k being relatively prime. (Two integers are said to be "relatively prime" if they have no common denominator other than 1.)
[Condition 2] Sub-scan feed amount F being identical with number of used nozzles N.
Printing speed increase and picture quality improvement are two major issues for the ink jet printers. The number of nozzles provided on a print head is to be increased to raise the printing speed In the interlace printing scheme, since the sub-scan feed amount F is set equal to the number of used nozzles N, the increase in the number of nozzles increases the sub-scan feed amount.
Mechanical accuracy of the sub-scan feed is, however, worsened substantially in proportion to the increase in sub-scan feed amount. The increase in the number of nozzles thus results in worsening the accuracy of the sub-scan feed. Especially when plural cycles of sub-scan feeds are carried out between recording of two adjacent raster lines, the feed errors due to the plural cycles of sub-scan feeds are accumulated and thereby significantly changes a pitch between the adjacent raster lines from a normal pitch. For example, in FIG. 16, three sub-scan feeds are carried out between the main scan of the second raster line and that of the third raster line. The pitch between these two raster lines is accordingly affected by the accumulated errors due to the sub-scan feeds.
FIG. 17 shows dots recorded in the conventional interlace printing scheme of FIG. 16. The pitch between the second raster line and the third raster line is increased by the accumulated errors due to the sub-scan feeds. This causes observable strip-like deterioration of the picture quality, which is called the "banding". Since the banding deteriorates the picture quality, it has been long demanded to reduce the occurrence of banding.